Lithium-7 is fragile, burning in stars at a relatively low
temperature. As a result, the majority of any interstellar
7Li cycled through stars is destroyed. For the same reason,
it is difficult for stars to create new 7Li and/or to return
any newly synthesized 7Li to the ISM before it is destroyed
by nuclear burning. In addition to synthesis in stars, the
intermediate-mass nuclides 6Li, 7Li,
9Be, 10B,
and 11B can be synthesized via cosmic ray nucleosynthesis,
either by alpha-alpha fusion reactions, or by spallation
reactions (nuclear breakup) in collisions between protons and
alpha particles and CNO nuclei. In the early Galaxy, when
the metallicity is low, the post-BBN production of lithium
is expected to be subdominant to that from BBN abundance.
As the data in Figure 9 reveal, only relatively
late in the evolution of the Galaxy does the lithium abundance
increase. The data also confirm the anticipated "Spite plateau"
(Spite & Spite
1982),
the absence of a significant slope in
the Li/H versus [Fe/H] relation at low metallicity due to the
dominance of BBN-produced 7Li. The plateau is a clear signal
of the primordial lithium abundance. Notice, also, the enormous
spread among the lithium abundances at higher metallicity.
This range in Li/H likely results from the destruction/dilution
of lithium on the surfaces of the observed stars while they are
on the main sequence and/or lithium destruction during their
pre-main sequence evolution, implying that it is the upper
envelope of the Li/H versus [Fe/H] relation that preserves the
history of Galactic lithium evolution. Note, also, that at low
metallicity the dispersion is much narrower, suggesting that
corrections for depletion/dilution are (may be) much smaller
for the Population II stars.

Figure 9. A compilation of the lithium
abundance data as a function of metallicity from stellar observations
(courtesy of V. V. Smith).
(Li)
1012(Li/H),
and [Fe/H] is the usual logarithmic metallicity relative to
solar. Note the "Spite plateau" in Li/H for [Fe/H]
-2.

As with the other relic nuclides, the dominant uncertainties
in estimating the primordial abundance of 7Li are not
statistical, but systematic. The lithium observed
in the atmospheres of cool, metal-poor, Population II halo stars
is most relevant for determining the BBN 7Li abundance.
Uncertainties in the lithium equivalent width measurements,
in the temperature scales for the cool Population II stars, and in
their model atmospheres dominate the overall error budget. For example,
Ryan et al. (2000),
using the
Ryan, Norris, &
Beers (1999)
data, infer [Li]P 12 +
log(Li/H) = 2.1, while
Bonifacio & Molaro
(1997) and
Bonifacio, Molaro, &
Pasquini (1997)
derive [Li]P = 2.2, and
Thorburn (1994)
finds [Li]P = 2.3. From
recent observations of stars in a metal-poor globular cluster,
Bonifacio et al. (2002)
derive [Li]P = 2.34 ± 0.056.
As may be seen from Figure 9, the indication
from the preliminary data assembled by V. V. Smith (private
communication) favors a Spite plateau at [Li]P 2.2.

In addition to these intrinsic uncertainties, there are others
associated with stellar structure and evolution. The metal-poor halo
stars that define the primordial lithium plateau are very old. As a
result, they have had time to disturb the prestellar lithium that could
survive in their cooler, outer layers. Mixing of these outer layers with
the hotter interior where lithium has been (can be) destroyed will
dilute or deplete the surface lithium abundance. Pinsonneault et al.
(1999,
2002)
have shown that rotational mixing may decrease the surface
abundance of lithium in these Population II stars by 0.1 - 0.3
dex while still maintaining the rather narrow dispersion
among the plateau abundances (see also
Chaboyer et al. 1992;
Theado & Vauclair
2001;
Salaris & Weiss
2002).
Pinsonneault et
al. (2002)
adopted for a baseline (Spite plateau) estimate
[Li] = 2.2 ± 0.1, while for an overall depletion factor
0.2 ± 0.1 dex was chosen. Adding these contributions to
the log of the primordial lithium abundance linearly,
an estimate [Li]P = 2.4 ± 0.2 was derived. In
the comparison between theory and observation below, I will adopt the
Ryan et al. (2000)
estimate [Li]P = 2.1 ± 0.1, but I
will also consider the implications of the
Pinsonneault et
al. (2002)
value.